PROJECT SUMMARY/ABSTRACT Over 140 million Americans are among the more than 500 million people who receive influenza vaccines an- nually. An important subgroup are healthcare workers (HCWs) for whom vaccination is recommended, and sometimes mandated, to protect themselves and vulnerable patients from influenza infection. However, there have been no large, long term studies of HCWs to support the effectiveness of these policies. HCWs are now a highly vaccinated population, the effects of which are also poorly understood. Mounting evidence suggests an- tibody responses to vaccination can be attenuated with repeated vaccination, which is corroborated by reports of poor vaccine effectiveness among the repeatedly vaccinated. Thus, there is a compelling need to directly evaluate HCW vaccination programs. The long term goal is to improve the efficient and effective use of influ- enza vaccines. The objectives of this application are to understand the long-term consequences of repeated annual influenza vaccination among HCWs and to use statistical and mathematical modeling to elucidate the immunological processes that underlie vaccination responses and their implications for vaccination effective- ness. These objectives will be achieved by pursuing three specific aims: 1) To study the immunogenicity and effectiveness of influenza vaccination by prior vaccination experience; 2) To characterize immunological pro- files associated with vaccination and infection; and 3) To evaluate the impact of immunity on vaccination effec- tiveness. Under Aim 1, a cohort of hospital workers will be recruited and followed for up to 4 years to assess their pre- and post-vaccination and post-season antibody responses, and their risk of influenza infection. These outcomes will be compared by vaccination experience, classified as frequently vaccinated (received ?3 vac- cines in the past 5 years), infrequently vaccinated (<3 vaccinations in past 5 years), vaccinated once, vaccine- nave and unvaccinated. In Aim 2, intensive cellular and serological assessments will be conducted to dissect the influenza HA-reactive B cell and antibody response, and build antibody landscapes that typify the different vaccination groups. In Aim 3, the data generated in Aims 1 and 2 will be used to develop a mathematical mod- el that considers prior infection, vaccination history, antibody kinetics, and antigenic distance to understand the effects of repeated vaccination on vaccine effectiveness. This approach is innovative because it will provide insights into the effect of complex immunological dynamics on infection outcomes, thereby representing a nov- el departure from previous studies, which have ignored these difficult-to-measure processes. Completion of the proposed research will provide evidence to inform decisions about continued support for influenza vaccination programs among HCWs and general policies for annual influenza vaccination, as well as much-needed clarity about the effects of repeated vaccination.